.XM File Format

Introduction

XM is the module format of FastTracker 2, a tracker for MS-DOS made by the group Triton. It extends the Amiga MOD idea with up to 32 channels, instruments that hold several samples across the keyboard, volume and panning envelopes, 16-bit samples and patterns of any length up to 256 rows.

XM was one of the main formats of the PC demo and tracker music scene, and modern trackers and players still read and write it.

Structure

An XM file starts with the text Extended Module: , the module name, the byte 1A, the tracker name and a version number, usually 0x0104. A header follows, whose size is stored first, with the song length, restart position, channel count, pattern count, instrument count, flags, default tempo and BPM, and a 256-byte pattern order table. Numbers are little-endian.

Each pattern has its own header (length, packing type, row count and packed data size) and then its notes. The notes are packed: a byte with its top bit set says which of the five fields (note, instrument, volume column, effect, effect parameter) follow, so empty cells take one byte. Instruments come last. Each has a header with its name and sample count, a keyboard map and envelopes, then its sample headers and sample data. Sample data is stored as differences between neighbouring values, which compresses well.

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